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  • JR-20 tone cabinet

    I just picked up a Hammond JR-20 tone cabinet for $50. A bit dusty inside but it looks like all the parts are there, it's got at least three 10" Jensens, and one oddball... I think it's a later "220" labeled alnico. On top where the 12" goes, there's a chunk of plywood with a smaller oval-shaped speaker attached (pointed up, not down for whatever reason). I have the original 12" Utah from my Leslie 120 that might be at home there.

    Anyway, once I get in there and give the innards the detail job they richly deserve, I'm going to want to try it out. Are there any considerations before I connect the 6-pin "tone cabinet" out on my A-102 to this bad boy?
    Hammond A-102 ('63), Leslie 122 ('63), Leslie 120 + mod horns, Yamaha SY-77, Technics KN2000, Fender USA Precision Bass (2016), Crown Micro-Tech 1200 x2

  • #2
    You said that it has a speaker pointed up? That speaker is usually pointed downward. It's the 12" treble speaker and yes, it points down, the purpose being to reflect the treble on the walls behind the cabinet so that the treble spreads across the room it's in. In reality, this is not the best sounding tone cabinet Hammond ever made, so lower your expectations.

    You should have 4 forward-facing 8" (?) speakers. Those 4 are in parallel and are the bass channel.

    The amplifier will need some attention if it's never been worked on. It may have some paper and wax capacitors, and also some metal-cased coupling capacitors that are leaky. If you are comfortable identifying and testing capacitors for leakage, please do so.

    Does your JR-20 still have the spring reverb system intact? If so, I hope it was transported in the upright position, or by now you have a nasty mess of oil everywhere. You may need to add a bit of mineral oil to the two long tubes and the short tube. The instruction card attached to the JR-20 and available in the Hammond Service Manual has more detail, as well as how to lock or unlock the spring mechanism during transportation.

    A common failure of these amps is the reverb drive transformer can open up. Check the resistance of the primary and secondary of the reverb drive transformer. I need to go back and check what the values ought to be, but most of my previous work on one is documented here: http://www.organforum.com/forums/sho...oins-the-flock

    There is no fuse installed on these, so I would strongly recommend adding an in-line 2 amp fuse to the incoming AC Hot wire, that feeds the power transformer primary.

    Check the tubes. The 6V6 tubes may have uneven wear and tear, and you want them to be sort of closely matched to one another.

    The giant oil block capacitors seem to never fail.

    Besides all of that, you can connect this to your A-102 using a 5 pin to 6 pin cable, attaching directly to the 5-pin output of your AO-39 amplifier, that is, assuming it has not been modified to be any other kind of output.

    More specific detail about the amp and its components can be dug up on this semi-ancient thread: http://www.organforum.com/forums/sho...636-JR-20-caps
    DISCLAIMER: When I started this thread in 2013 I was still getting my feet wet with tube amp servicing, so there is a lot of mis-steps I may have made on that particular amp, but you can see that the resistance of the reverb drive transformer is bout 750 ohms from center tap to each lead, but I'm getting ahead of myself.

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    • #3
      Fantastic! I'll be disassembling to do a thorough cleaning, so I'll check out the caps and transformer while it's in pieces. Bummer, I was thinking the AO-39 out was 6-pin because I have a couple extra Leslie cables. I guess I could modify one.

      I transported it on its back, but no oil spill... probably because the last guy did the same thing. I found the hang tag explaining the spring lock after I already got it home. Guy I got it from told me he does trashouts for a property management company. I asked him where was the organ it came with. He said probably gone... it was an organ and two cabinets, but he didn't have room to take all of it. I can't help but imagine a B3 and Leslie in a dumpster somewhere.
      Hammond A-102 ('63), Leslie 122 ('63), Leslie 120 + mod horns, Yamaha SY-77, Technics KN2000, Fender USA Precision Bass (2016), Crown Micro-Tech 1200 x2

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      • #4
        Why did you say “bummer”?

        The AO-39 stock has a 5 pin socket. The JR-20 amp has a 6 pin plug. Using a 5 pin (male) to 6 pin (female) cable, should interface fine with your A-102.

        If your A-102 is set up with a 6H type connector for a 122 type, you can use that 6 pin socket but don’t apply speed switching voltage to the tone cabinet.

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        • #5
          I meant bummer, I only have 6-pin to 6-pin cables. However, I do have a 6W connector... would that work?
          Hammond A-102 ('63), Leslie 122 ('63), Leslie 120 + mod horns, Yamaha SY-77, Technics KN2000, Fender USA Precision Bass (2016), Crown Micro-Tech 1200 x2

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          • #6
            No no no no no.

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            • #7
              Originally posted by muckelroy View Post
              No no no no no.
              The reason this doesn't work is the pin configuration is different. You'll send power to the wrong place and blow the amp. You need to make sure the right connections are made.
              When I become dictator, those who preach intolerance will not be tolerated.

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